1. Field of the Invention
The present invention relates in general to a toroidal type continuously variable transmission, and more particularly to a dual cavity toroidal type continuously variable transmission which includes a couple of toroidal type speed change units.
2. Description of the Prior Art
A toroidal type continuously variable transmission is a transmission using a toroidal type speed change unit as an essential element. The rotoidal type speed change unit comprises aligned input and output cone discs and power (or friction) rollers each being put between the input and output discs establishing a frictional engagement therewith. When a larger torque transmission is needed, a dual type unit including two toroidal type speed change units is commonly used such as one disclosed in Japanese Patent First Provisional Publication 4-69439. Usually, the transmission having such dual type unit is called a dual cavity toroidal type continuously variable transmission.
That is, in the dual type unit of the publication, two, that is, front and rear, toroidal type speed change units are coaxially arranged in a transmission case along a main shaft with their respective output discs arranged back-to-back with each other. The output discs are connected to rotate together about the main shaft. Respective input discs of the units are connected to the main shaft to rotate therewith. With a torque applied to the input disc of the front speed change unit, two input discs of the two speed change units are rotated together. A loading cam device is arranged about the main shaft in front of the front toroidal type speed change unit. With a thrust applied by the loading cam device to the input disc of the front speed change unit in accordance with a transmitted torque, the input discs of the two speed change units are biased toward the corresponding output discs. With this biasing force, power rollers of each speed change unit is compressed between the input and output discs with a force corresponding to the biasing force to assure power transmission from the input disc to the output disc. That is, power applied to each input disc is transmitted to the corresponding power rollers and then to the corresponding output disc. When, during this power transmission, the inclination angle of each power roller of each speed change unit is continuously changed relative to a rotation axis of the corresponding input and output discs, speed change is continuously carried out between the input and output discs. During this, the thrust applied to the respective input discs by the loading cam device biases the power rollers against the output discs.
In a dual cavity toroidal type continuously variable transmission including the two toroidal type speed change units, it is common to rotatably support the main shaft at the position between the respective output discs of the speed change units. For this supporting, there is provided in the transmission case a gear housing (or intermediate wall member) through which the main shaft passes in a manner to be rotatably supported thereby. To install the gear housing in the transmission case, the same is led into the case from a front open end of the transmission case and secured to a given portion of the case by bolts.
One arrangement for the gear housing in a transmission case is shown in Japanese Patent First Provisional Publication 5-39834. In this arrangement, an imaginary circle on which bolt holes for the bolts are arranged is positioned radially inside of an outer periphery of each output disc with respect to the main shaft. For this arrangement, it is inevitably necessary to take such assembling steps that at first, the gear housing is led into the transmission case and bolted to the given portion of the case and then the two output discs and associated parts are assembled to the gear housing. However, these assembling steps are troublesome and thus result in a greater assembly time. Furthermore, in the arrangement of the publication, it is somewhat difficult to obtain a precisely synchronized operation of the power rollers between the front and rear toroidal type speed change units. In fact, in such arrangement, it is difficult to provide around the gear housing with a suitable space through which an upper link bridging member needed for the synchronized operation passes.
In view of the above, the applicant has proposed a measure which is disclosed in Japanese Patent First Provisional Publication 7-158712 in which an imaginary circle on which the bolt holes are arranged is positioned radially outside of the outer periphery of each output disc. In this transmission, before inserting a gear housing into the transmission case, almost all parts to be mounted to the gear housing are preassembled to the gear housing and then the impregnated gear housing is bolted to the given portion of the transmission case. These assembling steps are simple. Furthermore, the arrangement of this publication can easily provide around the gear housing a certain space through which the upper link bridging member can pass.
As is known, the dual cavity toroidal type continuously variable transmission has, in addition to the above-mentioned two speed change units, a so-called forward/backward switching mechanism which is arranged at a front of the speed change units to transmit the rotation of an input shaft to the input discs while keeping or changing the direction of rotation.
The forward/backward switching mechanism comprises a planetary gear unit. Under power transmission in a normal direction, two rotatable members of the planetary gear unit are coupled by a forward clutch to permit the unit to assume a normally-coupled state. While, under power transmission in a reversed direction, one of the rotatable members is fixed to the transmission case by a backward brake to permit the unit to assume a reversely-coupled state.
In the forward/backward switching mechanism having the above-mentioned arrangement, driven plates of the backward brake are axially movably held by splines formed on an inner wall of the front open end of the transmission case. However, in order to permit insertion of the preassembled gear housing into the transmission case from the front open end, the splined portion of the inner wall has such a construction and size that tops of ridges of the splines are positioned radially outside of the above-mentioned imaginary circle. However, as is easily known, this arrangement tends to induce a bulky construction of the front open end of the transmission case, and thus, that of the transmission case.